High surface area nano-structured graphene composites and capacitive devices incorporating the same

ABSTRACT

A carbon composite material, including a plurality of spaced graphene sheets, each respective sheet having opposed generally planar surfaces, and a plurality of functionalized carbonaceous particles. At least some functionalized carbonaceous particles are disposed between any two adjacent graphene sheets, and each respective at least some functionalized carbonaceous particle is attached to both respective any two adjacent graphene sheets. Each respective graphene sheet comprises at least one layer of graphene and at least portions of respective any two adjacent graphene sheets are oriented substantially parallel with one another.

TECHNICAL FIELD

The novel technology relates generally to materials science, and, moreparticularly, to a high surface area graphene composite material.

BACKGROUND

Graphene, a single-atom-thick sheet consisting of sp² hybridized carbonatoms arrayed in a honeycomb pattern, is the building block of graphiticcarbons. Graphene may be viewed as an individual atomic plane of thegraphite structure. Graphene as a two-dimensional nanosheet hasattracted increasing interest due to its unique properties of highin-plane electronic conductivity, high tensile modulus, and high surfacearea, which make graphene an attractive candidate for applications inelectronic devices and composite materials. Moreover, with its highsurface area and good chemical stability, graphene may be used as a gasadsorbant, ultracapacitor material, or a supporting material fordeveloping novel heterogeneous catalysts with enhanced catalyticactivity.

Graphene may be produced by any one of several methods, including thestraightforward exfoliation technique of manually peeling off of the topsurface of small mesas of pyrolytic graphite, chemical vapor depositionon metal surfaces, epitaxial growth on electrically insulating surfaces,such as SiC, and the like. Although multiple production methods doexist, large-scale applications of graphene require simple and costeffective methods of production. Hence, the primary route in makinggraphene is still the exfoliation of graphite oxides followed by achemical reduction.

In aqueous solvent dispersions of graphene prepared by chemicalreduction, graphene sheets are separated by solvents stabilized byelectrostatic forces associated with ionizable groups introduced duringthe exfoliation. However, like other dispersions of nanomaterials withhigh aspect ratios, after the solvent is removed from the dispersion,the dried graphene sheets (GSs) usually aggregate and form anirreversibly interconnected or tangled precipitated agglomerate. Thisagglomeration is driven by the van der Waals interactions between theneighboring graphene sheets, urging the graphene sheets to stack backtogether in a disorganized and typically haphazard fashion. Thisagglomeration also leads to a considerable loss of the effective surfacearea of graphene, which affects the graphene applications in, forexample, supercapacitors, batteries, and catalyst supports, where a highsurface area of active materials is desired for performance. Therefore,how to achieve the intrinsically ultra-high surface area of graphene inits solid state is of interest in advancing the applications of graphenematerials.

Anchoring nanoparticles on the graphene surface before the GS'saggregation is one effective way to keep the GS's high surface area. Thedeposition of Pt nanoparticles on a graphene surface before drying hasbeen shown to increase the surface area of the composite from 44 m²/g to862 m²/g with the anchoring of the Pt nanoparticles on the surface.Graphene polyoxometalate nanoparticle composites have been observed toyield a graphene surface area of about 680 m²/g. Graphene sheet/RuO₂composites have been observed with increased surface area increases from108 m²/g to 281 m²/g. These composites also exhibited a high specificcapacitance 570 F/g and an enhanced rate capability. Although thesurface area of GSs have been increased with the addition of thenanoparticles, the resulting specific surface area was still much lowerthan the theoretical surface area of 2630 m²/g of the isolated GSs.

Thus, there is a need for graphene materials having effective surfacesareas approaching the theoretical maximum of 2630 m²/g. Further, thereremains a need for a method of reliably producing the same. The presentnovel technology addresses these needs.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the graphene sheet (GS) and thegraphene sheet nanocarbon composites (GSNC) preparation process.

FIG. 2 graphically illustrates nitrogen adsorption and desorption of theas-prepared GNCs with different nanocarbon content.

FIG. 3A illustrates TEM images of the as-prepared GSNCs from pure GSs.

FIG. 3B illustrates TEM images of the as-prepared GSNCs with 1%nanocarbon content and a surface area of 1256 m²/g.

FIG. 3C illustrates TEM images of the as-prepared GSNCs withfunctionalized nanocarbons.

FIG. 3D illustrates TEM images of the as-prepared GSNCs with 1%nanocarbon content and a surface area of 1256 m²/g.

FIG. 4A presents SEM images of the pure GSs.

FIG. 4B presents SEM images of GSNCs with a 1% nanocarbon content and asurface area of 1256 m²/g after drying.

FIG. 5A graphically illustrates CV curves of the as-prepared GSNCs witha surface area of 1256 m²/g, measured at potential intervals from −0.2to 0.8 V (vs. SHE) in 1 M H₂SO₄.

FIG. 5B graphically illustrates the capacitance of the GSNCs withdifferent nanocarbon content as the function of current density.

FIG. 6 schematically illustrates Pt nanoparticle etching process on thesurface of graphene sheets, according to another embodiment of thepresent novel technology.

FIG. 7A is a first atomic resolution electron micrographs showing thedynamic etching of graphene sheets by Pt nanoparticles and the resultingtrenches left behind in the graphene according to the embodiment of FIG.6.

FIG. 7B is a second atomic resolution electron micrographs showing thedynamic etching of graphene sheets by Pt nanoparticles and the resultingtrenches left behind in the graphene according to the embodiment of FIG.7A.

FIG. 7C is a third atomic resolution electron micrographs showing thedynamic etching of graphene sheets by Pt nanoparticles and the resultingtrenches left behind in the graphene according to the embodiment of FIG.7A.

FIG. 7D is a fourth atomic resolution electron micrographs showing thedynamic etching of graphene sheets by Pt nanoparticles and the resultingtortured path left behind in the graphene according to the embodiment ofFIG. 6.

FIG. 7E is a fifth atomic resolution electron micrographs showing thedynamic etching of graphene sheets by Pt nanoparticles and the resultingetch path left behind in the graphene according to the embodiment ofFIG. 7D.

FIG. 7F is a sixth atomic resolution electron micrographs showing thedynamic etching of graphene sheets by Pt nanoparticles and the resultingetch path left behind in the graphene according to the embodiment ofFIG. 7D.

FIG. 8A is an electron micrograph of pristine graphene.

FIG. 8B is an electron micrograph of Pt nanoparticles etched grapheneaccording to the embodiment of FIG. 6.

FIG. 9 graphically illustrates the XPS spectra of graphene before andafter Pt nanoparticulate etching, according to the embodiment of FIG. 6.

FIG. 10A graphically illustrates the N₂ adsorption isotherms and CO₂capture properties of graphene composites for graphene, Pt/Graphene, andPt/Graphene 800° C. at 77 K. P/P°, relative pressure; STP, standardtemperature and pressure.

FIG. 10B graphically illustrates the N₂ adsorption isotherms and CO₂capture properties of graphene composites for graphene, Pt/Graphene, andPt/Graphene 800° C. at 273 K; filled and open symbols representadsorption and desorption branches, respectively.

FIG. 11 is a schematic illustration of a supercapacitor using electrodesmade from the embodiment of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of thenovel technology, reference will now be made to the embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the novel technology is thereby intended, suchalterations and further modifications in the illustrated device, andsuch further applications of the principles of the novel technology asillustrated therein being contemplated as would normally occur to oneskilled in the art to which the novel technology relates.

According to a first embodiment of the present novel technology, asillustrated in FIGS. 1-5B, graphene sheets 10 were prepared by theexfoliation of graphite oxide (a layered material consisting ofhydrophilic oxygenated graphene sheets with oxygen functional groups ontheir basal planes and edges), such as in water to yield a colloidalsuspension of almost entirely individual graphene sheets 10. Nanosizedcarbon particles 15, typically carbon black particles 15, werefunctionalized with hydrophilic groups, such as —SO₃H (i.e., bisulfateor hydrogen sulfite), and the GSNCs 20 were prepared with differentloadings of the functionalized carbon black particles 25 by simultaneouschemical reduction of both the graphene oxides 30 and the functionalizedcarbon black particles 25 while in solution. Functionalization is theaddition of functional groups onto the surface of a material by chemicalsynthesis methods or the like, and the functional group added can besubjected to ordinary synthesis methods to attach virtually any kind ofcompound onto the material's surface. The nanosized functionalizedcarbon black particles 25 attached to the surface of the GSs 10 andserved as spacers to separate/support the neighboring GSs 10, whichprevented the haphazard restacking of the graphene sheets 10 into arandomly oriented solid particulate mass, and consequently, resulted inthe generation of increased surface area. The specific surface area ofthe composites 20 was 1256 m²/g, and a maximum specific capacitance of240 F/g was observed at a current density of 1 A/g. In addition,graphene sheet composite-based capacitors using this composite material20 for the electrodes exhibited enhanced rate capability, the maximumsustainable continuous or pulsed current output. The above improvedelectrochemical performance of the GSNCs 20 is a product of their highsurface area and high electronic conductivity of the GSs 10.

While the carbon nanoparticles 15 discussed herein are specificallycarbon black, other allotropes of carbon may be selected. Amorphouscarbon, glass carbon, coke, carbon graphitized to various degrees ofgraphitization, diamondlike carbon, and diamond may also be selected,with the electrical and physical properties of the resulting compositematerial 20 varying as a result.

In the synthesis of the GSNCs 20, the GSs 10 were obtained by in situchemical reduction of exfoliated graphene oxides 30. As shown in FIG. 1,the construction of the GSNCs involved the following steps: first,exfoliation 40 of graphite oxides, then, mixing 45 the graphene oxidesheets 30 and functionalized nanocarbons 25, and finally, chemicalreduction 50 of the mixture. The nanocarbons 15 were functionalized 55by the dizonium reaction, and the nanocarbons 25 are highly hydrophilicafter functionalization 55. Graphene oxide sheets 30 exist in the liquiddispersion 60. After reduction 50 of the compound 20 in its solid state,the graphene sheets 10 aggregate 65 and stack back into a layerstructure like graphite. Graphene oxide sheets 30 and carbonnanoparticles 25 exist together in dispersion 60; in the solid state thenanocarbons 25 serve as spacers, preventing the graphene sheets 10 fromrestacking back to the graphite structure, and thus make the graphenesheet 10 accessible on both sides and allowing access to the highsurface area graphene composite 20. In the reduction process 50, thewell-dispersed graphene oxide sheets 30 and the functionalizednanocarbons 25 were reduced simultaneously and the functionalizednanocarbon particles 25 became anchored 75 to the graphene sheets 10.The solid composites 20 float on the surface of the transparent liquidphase of the dispersion 60. The resultant graphene sheets 10 withattached functionalized nanocarbons 25 aggregated together to yield theGSNCs 20 upon drying.

Graphene oxides 30, possessing a considerable amount of hydroxyl andepoxide functional groups on both surfaces of each sheet 30, andcarboxyl groups, mostly at the sheet edges, are strongly hydrophilic andcan easily disperse in water. The nanocarbons 15 were functionalized 55by diazonium reactions as shown in FIG. 1A. In this process, thehydrophilic —SO₃H functional group was grafted onto the surface of thenanocarbons 15. As shown in FIG. 1, after functionalization 55 thenanocarbons 25 can disperse well in the water even if left for severalmonths. After adding the functionalized nanocarbons 25 into the graphenesheet dispersion 60, the two materials were able to be easily mixed andformed uniform dispersion 60.

In order to explore the effects of the nanocarbon content on thecomposite surface areas, a series of controlled experiments wereconducted by varying the content of the nanocarbon in the GSNCs 20 to 0,0.5, 0.8 and 1 wt. %. The addition of the nanocarbons 25 into thedispersion 60 of the graphene sheets 30 led to the formation ofwell-dispersed nanocarbon particles 25 on the surface of the graphenesheets 30. The in situ formed nanocarbon particles 25 can serve asspacers to prevent aggregation/restacking of the individual graphenesheets 30 in the dispersion during the drying process and form aparticle-sheet structured GSNC 20 in the solid state. It is reasonablyexpected that the in-situ-formed composites 20 have more of a richporous structure and large available surface area for the charge-storageprocess than those obtained by drying the pure graphene sheets 10, inwhich the restacking of the graphene sheets 10 inevitably occurs.

The nitrogen-adsorption and -desorption isotherms of the as-prepared GSs10 with different nanocarbon content exhibited type IV characteristics(FIG. 2), which are indicative of the presence of relatively large poresin the composites 20. It is worth noting that the Brunauer-Emmett-Teller(BET) specific surface area of the graphene sheets without the additionof nanocarbons (77 m²/g) was much lower than the theoretical predictionsfor the isolated graphene sheets (2630 m²/g). With the increase innanocarbon content in the composites 20, the specific surface area alsoincreased. The BET-specific surface area of the composites 20 withnanocarbon content of 1 wt. % reached as high as 1256 m²/g, which ismuch higher than that of the nanocarbons 25 (790 m²/g) and the pure GS10 (77 m²/g). In further trials, the BET-specific surface area of thecomposites 20 with additional nanocarbon material 25 was observed to beup to 1875 m²/g, and values as high as 2000, 2100 and approaching thetheoretical maximum are expected.

The large specific surface area suggests that the introduction ofnanocarbon particles 25 between 2D graphene sheets 10 effectively limitsthe face-to-face stacking from about forty layers of graphene sheets 10per stack to about two layers of graphene sheets 10 per stack whencompared with that of dried pure GS 80.

To further characterize the structure of the GSNCs 20, the samples wereexamined using transmission electron microscopy (TEM) and scanningelectron microscopy (SEM) (FIGS. 3 and 4). For comparison, the TEMimages of the reduced GSs 10 without nanocarbons 15 and thefunctionalized nanocarbons 25 (FIGS. 3A and 3D) are also presented. FIG.3A shows that the pure GSs 80 prepared by chemical reduction 50 weretransparent with some wrinkles visible under TEM. The morphology offunctionalized nanocarbons 25 can be seen in FIG. 3B, which shows thatthe functionalized nanocarbon particles 25 were in the range of 5-30 nm,and that they tended to spontaneously agglomerate together to form largeparticles. The structure of the GSNCs 20 is shown in FIG. 3C, whichclearly shows that the functionalized nanocarbons 25 were homogeneouslyanchored 75 onto the surface of the graphene sheets 30 (FIGS. 3C and3D). Through further comparisons of FIGS. 3A & 3B with FIGS. 3C & 3D, itis clear that the graphene sheets 30 served as substrates to anchor 75the hydrophilic nanocarbon particles 25. Without the addition of thenanocarbons, it can be seen that the pure GS 80 was less transparentthan the as-prepared GSNCs 20, because the pure graphene sheet 10spontaneously agglomerated/restacked back and formed a thick graphenesheet stack 80 (which has many more layers of graphene sheet than thatof the GSNC 20) after drying. The GSs 10 in the composites 20 werealmost transparent, which suggests that the GSs 10 were well separatedby the nanocarbon particles 25. The number of layers of graphene sheets10 in the composites 20 was lower (typically about two layers of GS 10,as suggested by BET data). Considering that sonication was used duringthe preparation of TEM specimens, the above observation alsodemonstrates the strong interactions/bonding between the nanocarboncarbon particles 25 and the graphene sheet 10 surface. The SEM imagesalso clearly show the difference between the pure GS agglomerations 80and the GSNCs 20. The pure GSs 10 after drying tended to restack andform solid particles 80 (FIG. 4A). However, the layered structure can beseen clearly for GSNCs 20, as the small nanocarbon particles 25 arehighly dispersed on the graphene sheet 10 surfaces and served as spacersto prevent the graphene sheets 10 from restacking, which is consistentwith the observed increased surface area of the graphenesheet/nanocarbon composites 20.

Recently, GS agglomerates 80 have been used as electrodes forsupercapacitors; for example, chemically modified GSs electrode activematerials in supercapacitors have been found to exhibit a specificcapacitance of 135 F/g and 99 F/g in aqueous KOH and organicelectrolytes, respectively. GS specimens 80 having a measured surfacearea of 534 m²/g have exhibited a capacitance of 150 F/g under thespecific current 0.1 A/g. Based on the structure of the GSNCs material20, the composite 20 likewise is expected to have good electronconductivity, low diffusion resistance to protons/cations, easyelectrolyte penetration, and high electroactive areas. Such composites20 are promising candidates for electrode active materials forsupercapacitors 100, yielding high performance energy storage devices.

The properties of these GSNCs 20 were measured using cyclic voltammetry(CV) and galvanostatic charge/discharge. The galvanostaticcharge/discharge was used to calculate the specific capacitance of theGSNCs 20. The CV curves (FIG. 5A) were nearly rectangular in shape,indicating a good charge propagation within the electrode. For thesupercapacitor 100 using the activated carbon-based electrodes 105, theCV curve shape and the specific capacitance significantly degraded asthe voltage scan rate increased. In contrast, as the scan rateincreased, the GSNCAs 100 base electrode 105 remained a rectangularshape with little variance, even at a scan rate of 200 mv/s (FIG. 5).Another indication of good charge propagation is the low variation ofspecific capacitance with the increase of the charge/discharge currentdensity as shown in FIGS. 5B and 5C. The capacitance of the GSNCs 20 was256 F/g at a discharge current density of 1 A/g, and the capacitance was218 F/g when the discharge current density increased to 5 A/g, leavingonly a 14.8% loss with the 400% increase on the discharge currentdensity. Therefore, the added nanocarbons played a very important rolein the electrochemical performance of the composites. The highperformance of the GSNC electrode materials 105 in the supercapacitor100 from the high surface area of this composite 20 is quite beneficial.

The specific capacitance of the GSNCs 20 with different amounts ofnanocarbons 25 at various current densities is shown in FIG. 5C forcomparison. It is worth noting that the specific capacitance of the highsurface area GS composites 20 was much higher than that of the pure GSs80, and the capacitance increased with the increase of the surface area.Hence, the increased surface area was responsible for the increase ofthe capacitance. The incorporation of nanocarbon particles 25 into theGSs 20 not only increased the surface area, but also acted as spacersbetween the graphene sheets 10 to create diffusion paths for the liquidelectrolytes, which facilitated the rapid transport of the electrolyteions, consequently resulting in the improved electrochemical propertiesof the GSNCs. Therefore, GSNCs 20 with a surface area of 1256 m²/gexhibited the maximum capacitance of 218 F/g at a current density of 5A/g compared with pure graphene materials 80 with a capacitance of 46F/g at the same current density, indicating that the unique structure ofthe novel GSNCs 20 facilitated the rapid transport of the electrolyteions and electrons throughout the electrode 105.

The simple process for preparing high surface area GSs 20 bysimultaneously reducing the graphene oxide sheets 30 and thefunctionalized nanocarbons 25 is more particularly described below. Thismethod is easily scaled up for the mass-production of high surface areagraphenes 20. The nanocarbon particles 25 are generally disperseduniformly on the surface of the graphene sheets 10, serving as spacersbetween graphene sheets 10, and preventing the restacking of the GSs 10after drying or removal of the solvent. Consequently, the GSNC surfacearea has been observed as high as 1875 m²/g. The unique structure of theGSNCs 20 facilitated the high-rate transportation of electrolyte ionsand electrons throughout the electrode 105, resulting in the excellentelectrochemical properties. The supercapacitor 100 based on the GSNCs 20exhibited a specific capacitance of nearly 400 F/g at a current densityof 1 A/g in a 1M H₂SO₄ solution. The specific capacitance increased withthe increase of the composite surface areas. The new high surface areaGS material 20 is also useful as a sorbent for hydrogen storage, as acatalyst support for fuel cells, and as a component for other cleanenergy devices.

Example 1 Synthesis of the Graphene Oxides (GO) and the FunctionalizedNanocarbons

GO 30 was synthesized from natural graphite powder (325 mesh) by themodified Hummer method. The GO 30 was then suspended 110 in water toyield an opaque dispersion 60, which was subjected to separation bycentrifuge (five times) to completely remove residual salts and acids.The purified GO 30 was then dispersed 120 in purified water (0.5 mg/mL).Exfoliation 40 of the GO 30 was achieved by ultrasonication of thedispersion 60 using an ultrasonic bath. During the composite preparationprocess, the number of single layers in the GSs 30 as a precursor aretypically controlled to be as small as possible. Graphite oxide is alayered material consisting of hydrophilic oxygenated GSs (grapheneoxides) 30 bearing oxygen functional groups in their basal planes andedges. Under appropriate conditions, graphite oxides can undergocomplete exfoliation in water, yielding colloidal suspensions 60 whereinthe suspended material is composed almost entirely of individualgraphene oxide sheets 30.

For the preparation of the nanosized carbon particles 25, the EC300carbon blacks 15 were modified with an —SO₃H grafted layer in an aqueousmedium by spontaneous reduction 50 of the corresponding in situgenerated diazonium cation. The modification of EC300 carbon blacks 15was prepared with a large excess of in situ-generated diazonium cations.In this experiment, 2 g of EC300 carbon blacks 15 were placed in a 0.5 MHCl solution 125 containing 3.5 g of sulfonic acid. The solution 125 wasvigorously stirred for thirty minutes before sodium nitrite was added.Next, 3.6 g NaNO₂ was added to the solution 125 in order to ensure atotal transformation of the amine into diazonium in spite of thenitrogen oxide gas release. For the reaction to be finished completely,the mixture was stirred for four hours and then heated up to 70° C. foranother three hours. Finally, the mixture was filtrated, washed withwater, and re-filtrated three times.

Synthesis of the GSNCs

GSNCs 20 with different nanocarbon content were prepared bysimultaneously reducing 50 the mixture of the graphene oxide sheets 30and the highly hydrophilic nanocarbons 25. Graphene oxide sheets 30dispersed in water were mixed with the nanocarbons 25. The mixture wasstirred for thirty minutes and then subjected to ultrasonication for onehour at room temperature. Subsequently, a hydrazine solution was addedinto the mixture and the mixture was stirred and heat treated at 100° C.for 24 hours. Then the mixture was filtered and washed with purifiedwater several times and dried at 60° C. for 24 hours in a vacuum.

Characterization of the Composites

The morphology of the graphene sheets 10, the nanocarbons 25, and theGSNCs 20 were characterized by a transmission electron microscope. Themorphology of the composites 20 was also examined by a scanning electronmicroscope. The specific surface areas of the graphene sheet 10, thenanocarbons 25, and the GSNCs 20 were measured by theBrunauer-Emmett-Teller (BET) method of nitrogen sorption at the liquidnitrogen temperature (77 K). Further, the composite materials 20 arestable at elevated temperatures and exhibit degradation or etching atthe nanocarbon particle 25 sites.

Preparation and Characterization of the Supercapacitor Electrode

A three-electrode-cell system was used to evaluate electrochemicalperformance using both cyclic voltammetry and galvanostaticcharge/discharge techniques using an electrochemical workstation. A 1MH₂SO₄ aqueous solution was used as the electrolyte. A platinum sheet anda saturated Ag/AgCl electrode were used as the counter and the referenceelectrodes, respectively. The working electrode 105 was prepared bycasting a Nafion-impregnated sample onto a glassy carbon electrode witha diameter of 5 mm. Next, 17.5 mg of composite material 20 was dispersedby sonication for ten minutes in a 10 mL water solution containing 54 ofa Nafion solution (5 wt. % in water). This sample (10 μL) was thendropped onto the glassy carbon electrode and dried overnight beforeelectrochemical testing. The specific gravimetric capacitance wasobtained from the discharge process according to the following equation:

$C = \frac{I\; \Delta \; t}{\Delta \; {Vm}}$

where I is the current load (A), Δt is the discharge time (s), ΔV is thepotential change during the discharge process, and m is the mass ofactive material in a single electrode (g).

Graphene 10 is generally quite inert when exposed to gases such asoxygen and hydrogen at room temperature. At high temperatures, oxygenexposure can cause preferential etching at defects and edges because thecarbon atoms at the defects and edges are extremely reactive (this isbecause the p_(z) electrons of these carbon atoms may not be involved inthe conjugated electron system). In a hydrogen atmosphere, the carbonatoms in the graphene bulk remain inert even at high temperatures.However, carbon atoms both at defects and at the edges of a graphenesheet become very active when a reactive metal is positioned proximateto these atoms. At high temperatures, Pt nanoparticles 150 may be usedto etch graphene 10 through the catalytic hydrogenation of carbon, wherecarbon atoms on the graphene edges dissociate on the surface of Ptnanoparticle 150 and then react with H₂ at the Pt nanoparticle 150surface to form methane. This process is shown schematically in FIG. 6.In contrast, such etching does not occur on graphene materials at carbonblack or like carbonaceous particle sites.

The mechanism of etching of graphene 10 by Pt nanoparticles 150 atelevated temperature was observed in-situ using high-resolutionenvironmental transmission electron microscopy. Graphene sheets 10 wereloaded with 20 weight percent of Pt nanoparticles 150, and subsequentlyplaced onto a lacey carbon TEM grid. The Pt nanoparticles 150 aretypically sized between a few nanometers up to ten microns across, andmay even be larger. More typically, the Pt nanoparticles are betweenabout 5 and about 80 nanometers in diameter, although the Ptnanoparticles 150 may more typically range from about 10 nanometers toabout 50 nanometers in diameter. The Pt nanoparticles 150 are typicallygenerally spherical, but may exhibit other morphologies. Further, thenanoparticles 150 may be made of PT-like materials, such as PT, Pd, Ni,combinations thereof, and the like. Likewise, in this example, thegraphene sheets 10 were loaded with 20 weight percent Pt nanoparticles150, but the nanoparticle loading may typically vary from less thanabout 1 weight percent to as much as 50 weight percent, or more. Thegraphene samples 10 were heated to 800° C. and hydrogen gas was slowlyintroduced into the TEM objective lens, and equilibrated at a pressureof approximately 50 mTorr. As the graphene 10 began to etch adjacent thePt nanoparticles 150, the process was imaged continuously through theuse of a high-frame rate camera. Image sequences extracted therefrom arepresented as FIG. 7A-7F. Initially, the Pt nanoparticles 150 were staticafter the hydrogen gas was introduced. Eventually, as shown in FIGS.7A-7C, the Pt nanoparticles 150 began to react with the graphene 10 atdefect sites and the hydrogen gas to produce methane. Only those carbonatoms making up the graphene sheet 10 that were in direct contact withthese Pt nanoparticles 150 were able to participate in this Pt-catalyzedhydrogenation reaction 155. Once the process was initiated, theconversion process was able to continue, as there are an abundance ofdefects sites created continuously following the onset of the etchingprocess 155, leading to a self-sustaining reaction. In this case astraight trench was etched through the graphene sheet 10 (FIG. 7C). Inother cases, the etching process 155 did not follow a straight line, butrather followed a more tortuous pathway (FIGS. 7D-7F). During thegraphene etching process 155, the Pt nanoparticles 150 were observed tomaintain a crystallographic relationship with the graphene sheet 10.After etching 155, the Pt nanoparticles 150 are typically reclaimed andsaved for future use. These observations indicate that the interactionbetween the Pt 150 and the graphene 10 at elevated temperature cancreate a variety of in-plane nanostructures 160 in the graphene 10. Theresult of these interactions is the formation of nanoscale trenches,ribbons and islands 160—and thus a dense network of edge sites 165.

Typically, the graphene sheets 10 are heated to a temperature sufficientfor the etching process 155 to occur at a desired rate. The graphenesheets 10 carrying dispersed Pt nanoparticles 150 are typically heatedto at least about 700 degrees Celsius, and are more typically heated toa temperature in the range from 750 degrees Celsius to 900 degreesCelsius. Likewise, a hydrogen gas environment supports the Pt-catalyzedhydrogenation reaction 155, although other reducing environments mayalso be selected.

In graphene 10, each carbon atom uses 3 of its 4 valance band (2s, 2p)electrons (which occupy the sp² orbits) to form covalent bonds with theneighboring carbon atoms in the same plane. Each carbon atom in thegraphene 10 contributes its fourth lone electron (occupying the p_(z)orbit) to form a delocalized electron system. Thus, the carbon atoms inthe graphene plane 10 (excluding the carbon atoms on the defect sitessuch as the edges and holes) are saturated carbon atoms, with the threesp² electrons forming three covalent bonds and the fourth p_(z) electronforming a π bond. Real time observations indicate that the heattreatment process creates an abundance of defective edges 165, in theform of embedded nanostructures of trenches, ribbons and islands 160 inthe multilayer graphene sheets 10 (see FIGS. 7 and 8B). The resultingmaterials are anisotropic, having different properties in-plane andout-plane. Importantly, the carbon atoms along the edges of theresulting trenches, ribbons, and the islands 160 are likely to beunsaturated, with one of the electrons in the sp² orbitals not forming acovalent bond with the other carbon atoms. These unsaturated carbonatoms were observed not only from the nano-scale in situ TEMimages/video (FIG. 7), but also from the macro-scale XPS results in FIG.9, which shows an 42.4% increase of the shake-up peak for the graphenesheet 10 after etching 155 (the shake-up peaks correspond to theunsaturated carbon atoms/dangling carbons).

The resulting material provides an important platform for a wide varietyof applications, including in catalysis, biomedical science, polymerscience and energy science. This is because these unsaturated carbonatoms allow graphene 10 to be functionalized by chemically graftingother compounds or groups thereonto. Thus, these functionalized graphene170 can be used, for example, sensors, catalysts, sorbents, and thelike. Without such features, it is difficult to chemically graftcompounds or groups onto graphene 10. These unsaturated carbons alsopromote the establishment of weak bonding between graphene and otherspecies. One such application is gas physisorption. Of particularinterest is the physisorption of carbon dioxide. The p_(z) electrons andone sp² electron of these unsaturated carbon atoms at the defects siteswill be available for bonding and will more readily form bonds with CO₂molecules, which could in turn result in a significant improvement inCO₂ adsorption. The adsorbed CO₂ molecules (or other gas molecules) maybe stored for later removal or reaction.

While the novel technology has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character. It is understood thatthe embodiments have been shown and described in the foregoingspecification in satisfaction of the best mode and enablementrequirements. It is understood that one of ordinary skill in the artcould readily make a nigh-infinite number of insubstantial changes andmodifications to the above-described embodiments and that it would beimpractical to attempt to describe all such embodiment variations in thepresent specification. Accordingly, it is understood that all changesand modifications that come within the spirit of the novel technologyare desired to be protected.

1. A carbon composite material, comprising: a plurality of spacedgraphene sheets, each respective sheet having opposed generally planarsurfaces; and a plurality of functionalized carbonaceous particles;wherein at least some functionalized carbonaceous particles are disposedbetween any two adjacent graphene sheets; wherein each respective atleast some functionalized carbonaceous particle is attached to bothrespective any two adjacent graphene sheets; wherein each respectivegraphene sheet comprises at least one layer of graphene; and wherein atleast portions of respective any two adjacent graphene sheets areoriented substantially parallel with one another.
 2. The compositematerial of claim 1 wherein the carbonaceous particles are substantiallyamorphous carbon.
 3. The composite material of claim 2 wherein thecarbonaceous particles are functionalized with —SO₃H radicals.
 4. Thecomposite material of claim 1 wherein the carbonaceous particles aregenerally spherical.
 5. The composite material of claim 4 wherein thecarbonaceous particles are between about 5 nm and about 80 nm indiameter.
 6. The composite material of claim 1 wherein the plurality ofgraphene sheets includes at least three graphene sheets.
 7. Thecomposite material of claim 1 wherein the composite material has aspecific surface area of at least about 800 m²/gm.
 8. The compositematerial of claim 1 wherein the composite material has a specificsurface area of at least about 1200 m²/gm.
 9. The composite material ofclaim 1 wherein the composite material has a specific surface area of atleast about 1800 m²/gm.
 10. The composite material of claim 1 whereinthe carbonaceous particles account for about 1 weight percent of thecomposite material.
 11. A method of manufacturing a high surface areagraphene composite material, comprising: a) substantially removingresidual salts and acids from graphite oxide to yield purified graphiteoxide; b) exfoliating the purified graphite oxide to yield graphiteoxide sheets; c) functionalizing carbonaceous particles with hydrophilicradicals; d) infiltrating graphite oxide sheets with functionalizedcarbonaceous particles; e) anchoring functionalized carbonaceousparticles onto graphite oxide sheets to yield particle-treated graphiteoxide sheets; and f) reducing particle-treated graphite oxide sheets toyield graphene sheets.
 12. The method of claim 11 and furthercomprising: g) drying graphene sheets; h) stacking dried graphene sheetsto yield a solid composite graphene material.
 13. The method of claim 11wherein the carbonaceous particles are generally spherical amorphouscarbon particles between about 5 nm and about 80 nm in diameter.
 14. Themethod of claim 11 wherein the carbonaceous particle treated graphenesheets have a specific surface area of at least about 800 m²/gm.
 15. Themethod of claim 11 wherein the carbonaceous particle treated graphenesheets have a specific surface area of at least about 1200 m²/gm. 16.The method of claim 11 wherein the carbonaceous particle treatedgraphene sheets have a specific surface area of at least about 1800m²/gm.
 17. An energy storage device, comprising: an energy storageportion; and two electrodes, each respective electrode operationallyconnected to the energy storage portion; wherein at least one electrodefurther comprises: a plurality of stacked generally parallel graphenelayers; and a plurality of functionalized carbon particles disposedbetween respective adjacent graphene layers for propping respectiveadjacent graphene layers apart.
 18. The energy storage device of claim17 wherein the functionalized carbon particles are generally sphericalamorphous carbon particles sized between about 5 nm and about 80 nm indiameter.
 19. The energy storage device of claim 17 wherein the at leastone electrode has a specific surface area of at least about 1800 m²/gm.20. The energy storage device of claim 19 wherein the capacitance of theat least one electrode is about 250 F/gm at a discharge current densityof about 1 A/gm and wherein the capacitance of the at least oneelectrode is about 220 F/gm at a discharge current density of about 5A/gm.
 21. An energy storage apparatus, comprising: an electrochemicalcell; two electrodes, each respective electrode operationally connectedto opposite poles of the electrochemical cell; wherein ions undergo aredox reaction at the surface of at least one electrode; wherein the atleast one electrode further comprises: a plurality of stacked graphenelayers; and a plurality of functionalized carbon particles disposedbetween respective adjacent graphene layers for propping the respectiveadjacent graphene layers apart.
 22. A method of etching graphene,comprising: identifying a graphene surface to be etched; positioningplatinum particles on the graphene surface; elevating the temperature ofthe graphene surface to about 800 degrees Celsius; placing the graphenesurface into a hydrogen gas environment; initiating removal of carbonatoms from the graphene surface through hydrogenation at defect sitesadjacent Pt nanoparticles; and etching embedded nanostructures into thegraphene surface.
 23. The method of claim 22 wherein the identifiedgraphene surface is part of a multilayered graphene structure defining aplurality of graphene layers separated by carbon particles positionedbetween respective graphene layers.
 24. A method for storing gas,comprising: identifying a multilayer graphene structure defining aplurality of graphene layers separated by carbon particles positionedbetween respective graphene layers, wherein the multilayer graphenestructure has a specific surface area of at least about 1200 m²/gm; andadsorbing gas onto the multilayer graphene structure.
 25. The method ofclaim 24 and further comprising: removing adsorbed gas from themultilayer graphene structure.
 26. The method of claim 24 and furthercomprising: reacting adsorbed gas on the multilayer graphene structure.